Abstract: To evaluate the effects on milk quality of supplementary Avoparcin in the diet, 40 Italian Friesian dairy cows were divided into two homogeneous groups, the treated one fed with Avoparcin added to a mineral-vitaminic premix, the control one fed with the same premix without Avoparcin. The trial lasted 60 days after two adapting weeks; each cow of the treated group received 120 mg daily of Avoparcin during the treatment period. No differences were found beetwen the two groups of cows for the following parameters: milk yield, total protein, fat, somatic cells and total bacterial count, while in the milk of the treated group lactose content was significantly (p<0.001) higher. After 45 days of treatment, trans isomers fatty acid content significantly increased (p<0.05) in the milk of the treated group, and particularly trans-vaccenic acid. Also some unsaturated fatty acids (linoleic and linolenic acid) significantly increased (p<0.05) their concentrations in milk fat of cows fed with Avoparcin supplementation.